What does the cuda option do in blender?
CUDA uses the CUDA cores of your GPU to do the rendering. In short, they're stream processors and do not affect how the output render looks. There is no difference between that and your GPU, in essence. I don't even think you can run an NVIDIA card without using CUDA cores for rendering for two reasons: 1. it may turn to molasses (analogous to running a PC on a USB ver. 1.0 instead of a harddrive), and 2. Blender won't know how. This is specific to NVIDIA. OpenCL is used for AMD cards. Since you have an NVIDIA card enabled, it will process the render. Otherwise, you're going to have to use your CPU. Do whatever is faster. I use a 635M and it is faster for me to use my Intel i5 3210 than the graphics card. You can read more about this here: https://docs.blender.org/manual/en/latest/render/cycles/gpu_rendering.html

Also this reference is good for explaining what CUDA cores are: https://www.lifewire.com/what-is-nvidia-cuda-834095